The study of protein structure and function has historically relied upon the reaction chemistries that are available using the reactive groups of the naturally occurring amino acids. Unfortunately, every known organism, from bacteria to humans, encodes the same twenty common amino acids (with the rare exceptions of selenocysteine (see, e.g., A. Bock et al., (1991), Molecular Microbiology 5:515-20) and pyrrolysine (see, e.g., G. Srinivasan, et al., (2002), Science 296:1459-62). This limited selection of R-groups has restricted the study of protein structure and function, where the studies are confined by the chemical properties of the naturally occurring amino acids.
The limiting number of natural amino acids restricts the ability to make highly targeted posttranslational protein modifications to the exclusion of all other amino acids in a protein. Most modification reactions currently used in the art involve covalent bond formation between nucleophilic and electrophilic reaction partners that target the naturally occurring nucleophilic residues in the protein amino acid side chains, e.g., the reaction of α-halo ketones with histidine or cysteine side chains. Selectivity in these cases is determined by the number and accessibility of the nucleophilic residues in the protein. Unfortunately, naturally occurring proteins frequently contain poorly positioned (e.g., inaccessible) reaction sites or multiple reaction targets (e.g., lysine, histidine and cysteine residues), resulting in poor selectivity in the modification reactions, making highly targeted protein modification by nucleophilic/electrophilic reagents difficult. Furthermore, the sites of modification are typically limited to the naturally occurring nucleophilic side chains of lysine, histidine or cysteine. Modification at other sites is difficult or impossible.
Alternative approaches for selectively modifying proteins with synthetic agents and probes, and covalent attachment of proteins to surfaces have been attempted. These include semisynthesis (Muir, Annu. Rev. Biochem. 2003, 72, 249-289), the use of electrophilic reagents that selectively label cysteine and lysine residues (Chilkoti et al., Bioconjugate Chem. 1994, 5, 504-507; Rosendahl et al., Bioconjugate Chem. 2005, 16, 200-207), and the selective introduction of amino acids with reactive side chains into proteins by in vitro biosynthesis with chemically aminoacylated tRNAs (Bain et al., J. Am. Chem. Soc. 1989, 111, 8013-8014; Ellman et al., Methods Enzymol. 1991, 202, 301-336). Each of these approaches suffers from either a lack of target specificity or other impracticalities.
One strategy to overcome the limitations of the existing genetic repertoire is to add amino acids that have distinguishing chemical properties to the genetic code. This approach has proven feasible using orthogonal tRNA molecules and corresponding novel orthogonal aminoacyl-tRNA synthetases to add unnatural amino acids to proteins using the in vivo protein biosynthetic machinery of a host cell, e.g., the eubacteria Escherichia coli (E. coli). This approach is described in various sources, for example, Chin et al., Science (2003) 301:964-967; Zhang et al., Proc. Natl. Acad. Sci. U.S.A. 2004, 101:8882-8887; Anderson et al., Proc. Natl. Acad. Sci. U.S.A. 2004, 101:7566-7571; Wang et al., (2001) Science 292:498-500; Chin et al., (2002) Journal of the American Chemical Society 124:9026-9027; Chin and Schultz, (2002) ChemBioChem 11:1135-1137; Chin, et al., (2002) PNAS United States of America 99:11020-11024; Wang and Schultz, (2002) Chem. Comm., 1-10; Wang and Schultz “Expanding the Genetic Code,” Angewandte Chemie Int. Ed., 44(1):34-66 (2005); and Xie and Schultz, “An Expanding Genetic Code,” Methods 36:227-238 (2005). See also, International Publications WO 2002/086075, entitled “METHODS AND COMPOSITIONS FOR THE PRODUCTION OF ORTHOGONAL tRNA AMINOACYL-tRNA SYNTHERASE PAIRS;” WO 2002/085923, entitled “IN VIVO INCORPORATION OF UNNATURAL AMINO ACIDS;” WO 2004/094593, entitled “EXPANDING THE EUKARYOTIC GENETIC CODE;” WO 2005/019415, filed Jul. 7, 2004; WO2005/007870, filed Jul. 7, 2004; WO 2005/007624, filed Jul. 7, 2004; and International Publication No. WO2006/034332, filed on Sep. 20, 2005.
Phage display technology is a malleable and widely utilized technique that has found applications in diverse biological disciplines. See, e.g., Smith and Petrenko, Chem. Rev., 97:391-410 (1997); Sidhu, Bimolecular Engineering 18:57-63 (2001); Rodi and Makowski, Current Opinion in Biotechnology 10:87-93 (1999); and Willats, Plant Molecular Biology 50:837-854 (2002). For example, phage display has proven very useful for the isolation of high-affinity ligands and receptors from large polypeptide libraries. It has the advantages that large libraries can be easily generated by recombinant methods, library members can be amplified for iterative rounds of enrichment, and primary structure can be determined by DNA sequencing. However, like proteins in general, phage-displayed peptide libraries are also restricted to the common 20 amino acid building blocks, limiting the functional groups that can be targeted for posttranslational modification. Moreover, methods for posttranslational modification of phage-displayed polypeptides, where the modification reaction uses physiologically-compatible conditions that preserve protein activity and phage viability present even greater challenge (Leieux and Bertozzi (1998) TIBTECH, 16:506).
In an attempt to expand the scope of phage-display utility, Noren and co-workers incorporated selenocysteine into phage displayed peptides using a natural selenocysteine opal suppressing tRNA (Sandman et al., J. Am Chem. Soc. (2000) 122:960-961). Roberts et al. attempted to generalize this approach to peptide libraries containing other unnatural amino acids using in vitro mRNA display (Li et al., J. Am. Chem. Soc., (2002) 124:9972) with chemically aminoacylated amber suppressor tRNAs (Noren et al., Science (1989) 244:182-188). However, the generation of a large number of such tRNAs is impractical, and they are consumed stoichiometrically.
What is needed in the art are new strategies for incorporation of unnatural amino acids into phage-displayed polypeptides for the purpose of modifying and studying protein structure and function, where the unnatural amino acids in the displayed polypeptides can be selectively targeted for posttranslational modification while displayed on the phage. There is a need in the art for the creation of new strategies for protein modification reactions that modify phage-displayed proteins in a highly selective fashion, and furthermore, allow the modification of the phage-displayed proteins under physiological conditions that preserve phage viability following the modification reaction. What is needed in the art are novel methods for producing targeted protein modifications on phage-displayed proteins, where the modifications are highly specific, e.g., modifications where none of the naturally occurring amino acids in the polypeptides are subject to cross reactions or side reactions. The invention described herein fulfills these and other needs, as will be apparent upon review of the following disclosure.